1. Field of the Invention
The present invention relates to a solid-state image sensing device, and more specifically to a method of driving a solid-state image sensing device having field memories adjoining a photosensitive region and being capable of reading two-line signal charges simultaneously.
2. Description of the Prior Art
Among the solid-state image sensing devices including a charge transfer device, a CCD area sensor, for instance, can obtain high resolution video signals. As a result, sensor is widely used as broadcasting CCDs, high picture quality CCDs, electronic still camera CCDs, etc.
In the conventional solid-state image sensing device used as an area sensor which can read two-line image signals simultaneously, a first accumulation region and a second accumulation region are formed on both sides of a photosensitive region, respectively, as field memories. Furthermore, in at least one of the accumulation regions, a cyclic transfer path where a plurality of transfer stages are connected into a loop circuit is provided so that the arrangement sequence of photosensitive elements (pixels) can be changed. The signal charges accumulated in these accumulation regions are transferred along a first horizontal transfer path and a second horizontal transfer path, respectively, and further are detected by a first charge detecting circuit and a second charge detecting circuit, both provided at the transfer ends, respectively. Thus, in general, the signal information (charges) of all the vertical pixels can be obtained within a single field period.
The method of driving the solid-state image sensing device as described above will be described hereinbelow. In the vertical transfer path of the above-mentioned image sensing device, since a single transfer stage is composed of two photosensitive pixels arranged in the vertical direction, the signal charges are read as follows: First, within a vertical blanking period (VBL) of odd field, in response to a first field shift (FS) pulse, the signal charges are read from a first pixel group (composed of photosensitive pixels of odd ordinal numbers counted from above in the vertical direction) to the vertical transfer path. Next, in response to a first field transfer pulse, the read signal charges are further transferred to a first accumulation region formed on one side of the photosensitive region. After that, in response to a second field shift (FS) pulse, the signal charges are read from a second pixel group (composed of photosensitive pixels of even ordinal numbers counted from above in the vertical direction) to the vertical transfer path. Further, in response to a second field transfer pulse, the read signal charges are further transferred to a second accumulation region formed on the other side of the photosensitive region.
In the transfer operation, in the first accumulation region, the signal charges are transferred cyclically by a pulse in such a way that the order of the photosensitive pixels arranged in the vertical direction can be reversed. Further, in synchronism with the horizontal scanning, the signal charges in the first and second accumulation regions are transferred stage by stage to the first and second horizontal transfer paths respectively, and then outputted within an effective horizontal blanking period (HBL) from the first and second charge detecting circuits, respectively, as electric time-series signals. The above-mentioned operation is repeated within the succeeding vertical blanking (VBL) of even field. Further, prior to the first FS pulse, a sweep-off transfer pulse is given to the image sensing device in order to eliminate false signals such as smear or dark current in the vertical transfer paths.
In the method of driving the above-mentioned solid-state image sensing device, since the signal charges read from the photosensitive pixels, arranged in the odd or even ordinal numbers counted from above in the vertical direction of the photosensitive region, are always outputted through different detecting circuits, there exist an imbalance in gain and frequency characteristics between the detecting circuits due to dispersion in the manufacturing process or a difference in circuit design. Thus, a problem arises in that there exists a difference in output between the scanning lines within the same field or between the different fields.